Houseplant · Euphorbiaceae
Pencil Cactus
Euphorbia tirucalli
A thicket of slim green stems, sun-loving and tough, whose milky sap bites back, so handle it with care.
Full sun
Every 14–28 days
50–86°F · 10–30°C
USDA 10–12
Overview
Up close the pencil cactus is a dense cluster of slim, cylindrical stems about as thick as a pencil, smooth and rounded and colored a plain, even green. Despite the name it is no cactus but a succulent spurge from East and southern Africa, and it carries almost no leaves, so the bare green stems do the work and give the whole plant its branching, twiggy shape. The stems fork and cross into a tangle that looks like a bundle of green sticks.
Break or cut one of those stems and it wells up at once with a caustic milky latex, a thick white sap loaded with diterpene esters that beads at the wound and runs. It burns skin and can badly injure the eyes, and that single feature, hidden inside every ordinary-looking stem, is the one to keep in mind whenever a hand goes near it.
Left to grow, the green stems stack and lengthen until the plant stands 2 to 10 feet (60 cm to 3 m) tall, thickening into a substantial mass over the years. Strong light keeps the stems stout and firm, a gritty fast-draining pot keeps the roots dry, and long gaps between waterings suit it, with a soft, rotted stem the main sign of too heavy a hand.
Care
The short version: full sun or the brightest window you have, a gritty fast-draining mix, and a deep soak only once the soil is bone dry. Ease off in winter, and always wear gloves and eye protection when you touch the stems.
Light
Give it full sun at a south-facing window, or the brightest indirect light you can manage. Under strong light the green stems stay stocky, thick, and firm, while a dim corner draws them out thin and pale and stretched.
Water
Let the mix dry out completely, then water deeply, which usually lands around every 14 to 28 days. Cut right back in winter. Standing moisture is what reliably kills this plant, turning the firm green stems and roots soft with rot.
Soil
Plant it in a gritty, fast-draining cactus or succulent mix so water runs straight through and never sits at the roots. A pH anywhere from 6.0 to 7.5 suits it, and a pot with a drainage hole is not optional.
Temperature
It sits happily between 50°F and 86°F (10°C to 30°C) and takes no frost. Keep it above 50°F (10°C) through winter, since cold paired with damp soil is what softens the stems into rot.
Humidity
Low household humidity is exactly what it wants, so there is no number to chase and no misting to do. The dry air actually helps, keeping the green stems firm and free of rot.
Feeding
Feed it monthly through spring and summer with a dilute balanced or low-nitrogen cactus fertilizer. Stop completely in autumn and winter while growth slows, and skip feeding a plant that is stressed or just repotted.
Pruning
Prune only to control size or shape, and never without gloves and eye protection, because a cut stem bleeds caustic white latex that burns skin and can injure the eyes. Cut into a bag, keep the plant away from your face, and wash your tools and hands the moment you finish.
Repotting
Repot every 2 to 3 years or once roots circle the pot and crowd the drainage holes. A top-heavy plant that tips easily is another sign. Wear gloves and eye protection, since repotting almost always nicks a stem and starts the white sap flowing.
Size & spacing
Indoors the green stems climb to 2 to 10 feet (60 cm to 3 m) tall and the tangle spreads 2 to 6 feet (60 to 180 cm) wide over time. Give it room to lean and grow, and set a tall specimen in a heavy pot so it does not topple.
Troubleshooting
This is a tough plant, so trouble almost always shows in the stems and comes down to one of two things: too much water or too little light. Look closely at their color and firmness, since that is where the first signs appear. Match what you see below and act before a soft patch spreads through the green.
Soft, mushy stems collapsing at the base
When firm green stems turn soft, yellowed, and slick to the touch near the soil, that is rot setting in, and it traces to water sitting in a mix that drains too slowly, hurried on by cold and damp. Pressed, the tissue gives and the base may fold and smell foul. Wearing gloves and eye protection, cut back to firm green flesh, let those sound pieces callus, and set them into dry, gritty mix.
Fix:
- Cut away all soft, discolored tissue back to firm green growth using gloves and eye protection
- Allow healthy cuttings to callus, then replant in dry, gritty mix
- Withhold water until the plant is clearly established and the mix is dry
Thin, pale stems stretching toward the window
Stems that grow thin, washed pale, and stretched out are showing etiolation, the plant straining toward light it cannot reach. The fresh growth arrives spindly and narrow and tips visibly toward the brightest source nearby. Move it into several hours of direct sun, stepping the light up slowly so the pale new tissue does not scorch.
Fix:
- Move to a brighter location with several hours of direct sun
- Acclimate gradually to stronger light to avoid scorch
Toxicity
Handle this plant as a real hazard rather than an ordinary houseplant. Inside every smooth green stem sits a caustic milky latex that burns skin into a stinging rash and, in the eyes, causes searing pain, swelling, and even temporary blindness. No part is edible, and the white sap is toxic to cats and dogs and irritating to every other animal, so keep the stems well away from pets, children, and bare hands.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Toxic
A cat that bites into a stem meets the thick white latex and starts drooling and pawing at its mouth, sometimes vomiting from the burn. Smeared onto the fur, the sap leaves the skin beneath red and sore, and a swipe near the eyes brings painful swelling and damage.
Dogs Toxic
Chewing a stem coats a dog's mouth in milky, burning sap, bringing drooling, pain, and a churning stomach. The latex raises blisters along the gums and lips, and any that reaches the eyes burns sharply enough to need a vet at once.
Horses Toxic
The caustic white sap inflames a horse's mouth and gut when it browses the stems. Where the latex touches, the muzzle and lips are left sore, reddened, and irritated.
Rabbits Toxic
Even a small bite through a stem releases enough stinging latex to inflame a rabbit's mouth and gut. These grazers gnaw at anything new, so the plant belongs well out of their reach.
Livestock Toxic
Goats, sheep, and cattle that browse the stems take in burning latex that scorches the mouth and unsettles the stomach. A fence between them and the plant keeps roaming animals clear.
Birds Toxic
The caustic sap stings a bird's beak, mouth, and crop. A bird that pecks a stem can smear the sticky white latex across its own feathers and eyes.
People Caution
No part is safe to eat, and the milky white sap is the real hazard. On skin it raises a stinging rash, and in the eyes it brings intense pain, swelling, and even temporary blindness, so gloves and eye protection go on before any handling.
Always wear gloves AND eye protection when handling, cutting, or repotting. Never touch your face or rub your eyes. If latex contacts skin, wash thoroughly; if it contacts the eyes, flush immediately with copious water and seek urgent medical or veterinary care.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
The plain species keeps its stems an even green, but one warm-toned cultivar turns up on shop shelves far more often.
'Firesticks' / 'Sticks on Fire' (Euphorbia tirucalli 'Rosea')
Its young stems flush warm coral, orange, and red along their tips rather than staying plain green, and those colors turn deeper and richer under cool weather and strong sun.
How to propagate
Pencil cactus grows readily from lengths of its own green stem, so one plant soon yields several. The catch is the white sap that wells from every cut, which makes careful handling the heart of the job.
Stem cuttingsEasy
Late spring to summer during active growth · 3 to 6 weeks to root
Cut a slim green stem in late spring or summer while the plant is growing and it weeps caustic white latex at once, so wear gloves and eye protection. Rinse the beaded sap from the cut face, let it dry to a firm callus over several days, then stand the length in dry, gritty mix, where it puts out roots in 3 to 6 weeks.
Myths
The pencil cactus is a true cactus.
For all its bare, spiny-looking stems, this is no cactus. Euphorbia tirucalli is a pencil-stemmed succulent spurge, and the tell is the caustic latex that wells up white from any wound, something a true cactus never bleeds. The likeness lives only in the silhouette.